NAND endurance is something that always raises questions among those considering a move to solid state storage. Even though we have showed morethanonce that the endurance of today's MLC NAND based SSDs is more than enough for even enterprise workloads, the misconception of SSDs having a short lifespan still lives. Back in the day when we had 3Xnm MLC NAND with 5,000 P/E cycles, people were worried about wearing our their SSDs, although there was absolutely nothing to worry about. The move to ~20nm MLC NAND has reduced the available P/E cycles to 3,000, but that's still plenty.

We have tested MLC NAND endurance before but with the release of Samsung SSD 840, we had something new to test: TLC NAND. We have explained the architectural differences between SLC, MLC and TLC NAND severaltimesbynow, but I'll do a brief recap here (I strongly recommend reading the detailed explanation if you want to truly understand how TLC NAND works):

SLC

MLC

TLC

Bits per Cell

1

2

3

P/E Cycles (2Xnm)

100,000

3,000

1,000

Read Time

25us

50us

~75us

Program Time

200-300us

600-900us

~900-1350us

Erase Time

1.5-2ms

3ms

~4.5ms

The main difference is that MLC stores two bits per cell, whereas TLC stores three. This results in eight voltage states instead of four (also means that one TLC cell has eight possible data values). Voltages used to program the cell are usually between 15V and 18V, so there isn't exactly a lot room to play with when you need to fit twice as many voltage states within the same space. The problem is that when the cell gets cycled (i.e. programmed and erased), the room taken by one voltage state increases due to electron trapping and current leakage. TLC can't tolerate as much change in the voltage states as MLC can because there is less voltage headroom and you can't end up in a situation where two voltage states become one (the cell wouldn't give valid values because it doesn't know if it's programmed as "110" or "111" for example). Hence the endurance of TLC NAND is lower; it simply cannot be programmed and erased as many times as MLC NAND and thus you can't write as much to a TLC NAND based SSD.

No manufacturer has openly wanted to discuss the endurance of TLC, so the numbers we have seen before have been educated guesses. 1,000 - 1,500 P/E cycles is what I've heard for TLC NAND. The reality can also be different from what manufacturers claim as we discovered in the Intel SSD 335 (though there is a high probability that it's just a firmware bug), so actually testing the endruance is vital.

There was one obstacle, though. Samsung does not report NAND writes like Intel does and without NAND writes we can't know for sure how much data is written to the NAND because of write amplification. Fortunately, there is a a workaround: I wrote incompressible 128KB sequential data (QD=1) to the drive and took down the duration of each run and the Wear Leveling Count (similar to Media Wear Indicator). If I know the average write speed and the duration, I can figure out how much I wrote to the drive. Sequential large block-size data should also result in write amplification near 1x because the data is sequential and thus doesn't fragment the drive. I then compared the amount of data I wrote to the WLC values I had recorded:

Samsung SSD 840 (250GB) Endurance Testing

Total Amount of Data Written

92,623 GiB

Total Amount of WLC Exhausted

34

Estimated Total Amount of P/E Cycles

1,064

Estimated Total Write Endurance

272,420 GiB

It seems that 1,000 P/E cycles is indeed accurate. The raw Wear Leveling Count seems to indicate the amount of exhausted P/E cycles as it's inversely proportional to the normalized WLC value and once it hits 1,000, the WLC will hit zero.

Note that if Samsung's WLC is anything like Intel's Media Wear Indicator, when the normalized counter value drops to 0 there's still a good amount of endurance actually left on the NAND (it could be as high as another 20 - 30%). At least with Intel drives, the MWI hitting 0 is a suggestion that you may want to think about replacing the drive and not a warning of imminent failure.

Conclusions

1,000 P/E cycles may not sound much but when it's put into perspective, it's still plenty. Client workloads rarely exceed 10GiB of writes per day on average and write amplification should stay within reasonable magnitudes as well:

SSD Lifetime Estimation

NAND

MLC—3K P/E Cycles

TLC—1K P/E Cycles

NAND Capacity

128GiB

256GiB

128GiB

256GiB

Writes per Day

10GiB

10GiB

10GiB

10GiB

Write Amplification

3x

3x

3x

3x

Total Estimated Lifespan

35.0 years

70.1 years

11.7 years

23.4 years

Of course, if you write 20GiB a day, the estimated lifespan will be halved, although we are still looking at several years. Even with 30GiB of writes a day the 256GiB TLC drive should be sufficient in terms of endurance. Write amplification can also go over 10x if your workload is heavily random write centric, but that is more common in the enterprise side - client workloads are usually much lighter.

Furthermore, it should be kept in mind that all SMART values that predict lifespan are conservative; it's highly unlikely that your drive will drop dead once the WLC or MWI hits zero. There is a great example at XtremeSystems where a 256GB Samsung SSD 830 is currently at nearly 6,000TiB of writes. Its WLC hit zero at 828TiB of writes, which means its endurance is over seven times higher than what the SMART values predicted. That doesn't mean all drives are as durable but especially SSDs from NAND manufacturers (e.g. Intel, Crucial/Micron, Samsung etc.) seem to be more durable than what the SMART values and datasheets indicate, which isn't a surprise given that they can cherry-pick the highest quality NAND chips.

I don't understand how you can assume SMART values are worse than actual ones? How many times has a product had a problem when it's "supposed" to be working.

This is something Anandtech should test for themselves, rather than take the manufacturer at their word. Do a continuous write test and see how many weeks it takes to fail. I'm pretty sure you'll encounter some surprises. Reply

A premature death is always a possibility but usually that's not due to the NAND endurance.

Totally killing the drive is obviously the best way to test endurance but the problem is that it will take weeks, possibly even months to complete and I couldn't test any other SSDs during that period.Reply

Just setup a cheap box with SATA3 and let it run. Why does it have to occupy the main test rig? After doing the main benchmarks stick the drive in the torture chamber. It would add a lot of value to reviews.

Remember when the stuttering problem was first discovered? It took some extra effort but it was worth it, and drive manufacturers ended up changing their products as a ersultReply

I don't think you understand which SMART values are being talked about here. Sure, if SMART doesn't find a problem with an HDD it doesn't have to mean anything. But if it finds a problem, you can be sure there is a problem.

Here, however, we're talking about the amount of data written to the drive logged and reported via SMART. And the manufacturer is calculating the wear level indicator from these values. However, they have to be conservative here - a dying drive still showing 50% SSD life left would get them into troube. Hence real world write endurance will be better than the wear indicator reported by SMART implies (as that link at XS shows).Reply

Your calculations look nice but are they really how SSDs will get used in real life?If you have a 128GB SSD with your OS and programs installed on it, you probably have used about 60GB. Then load some other data on it or even games and let's say you're left with a generous 20GB free space on the SSD.Most of the occupied space in the NAND won't change because system/prgram files don't get changed at all.So you probably only change 10GiB per day as you said, but the 60GB system files won't get changed, ever.So if data gets changed it will occur on the latter 60GB.Because 'only' 20GB are free, wear leveling will have to use the remaining 20GB most of the time.So finally the first 60GB won't wear at all, whereas the latter 60GB will get written to all the time, and the last 20GB will wear the fastest.

And because SSDs are expensive people tend to not waste space on a SSD, so I think 20GB free is luxury.

Or how do SSDs handle this? Reshuffle the content? But this causes additional writes and wear, too. So I think TLC SSDs lifespan is much much shorter than 12 years, maybe just the 2 or 3 years most people keep their computers nowadays.Reply